Signature of long-term class evolution in GRS 1915+105 at a high accretion rate

Abstract

We find long-term evolution of the ω class from the study of X-ray timing and spectral analysis of the Galactic microquasar GRS 1915+105 during two outburst activities, observed by the proportional counter array (PCA) and the High Energy X-ray Timing Experiment on-board Rossi X-ray Timing Explorer. The class is characterized by unusual periodic-like variation in intensity. With the passage of time, the variability gradually disappears and the class stabilizes at higher intensity. Low-frequency (0.1–10 Hz) power density spectral evolution and energy spectral evolution show that the class initially occurs as an unstable and oscillatory stage between the high-soft state and the low-soft/hard-intermediate state and finally settles in the high-soft state. This modulation process gives a clear signature of the adjustment of the accretion disc during an increasing accretion rate. Spectral analysis of this class at different stages shows a distinct feature of a single thermal Comptonization component along with a strong hard X-ray tail. We try to fit different observational characteristics found during the timing and spectral evolution with different disc accretion models and we find that the presence of an extended, optically thin corona coupled with an accretion disc can explain our results adequately. The thermal electron cloud covers a large fraction of the inner disc region, and it is powered by the accretion energy of the accretion disc. The change in the coronal temperature and optical depth between dip and non-dip regions of the light curve shows the change in the underneath disc emissivity at nearly constant disc temperature. When a high state is achieved, the corona becomes cool and optically thick and emission is dominated by a disc blackbody component. The strong support for the presence of an accretion-powered Comptonizing medium is the detection of delay in arriving time between soft and hard photons during this class. Both All-Sky Monitor and PCA flaring intensity and the nature of evolution are found to be the same for both X-ray outbursts at different time. This indicates that this process has fundamental contribution to the overall disc accretion mechanism during the outburst and is hence important in understanding the possible ways for utilizing the total accretion energy budget.